One other thing to check is the line inside the tank going to the clunk. It may have a pinhole leak in it somewhere that will cause this problem only after the tank level has gotten down. I never use the cheap line that comes with a tank when you get it. that pinhole in the fuel line will cause it to run lean only after you have been flying for awhile.

I 2nd the idea of strapping a different tank to the outside of the plane to see if you can see any foaming issues.